1. Field of the Invention.
This invention relates to an orthopedic prosthesis and more particularly to a prosthesis employing screws and bone cement for securing the prosthesis to a bone.
2. Description of the Prior Art.
Prior art orthopedic prostheses are used to replace damaged joints or to repair damaged bones. Surgery to implant an orthopedic prosthesis typically requires adjacent areas of the bone to be surgically resected. The resection is carried out to ensure that the bone and the prosthesis have compatible mating shapes and to provide appropriate support for the prosthesis. Orthopedic prostheses also must provide a connection between the bone and the prosthetic component to prevent relative shifting.
The prior art connections between orthopedic prostheses and bones have taken many forms. For example, some orthopedic prostheses are implanted by forming a cavity in the bone. The prosthetic component is configured for wedged engagement into the cavity. Other orthopedic prostheses include a porous coating on surfaces of the prosthetic component that will be in face-to-face mating engagement with the bone. The porous coating is intended to promote bone ingrowth. Many other prior art orthopedic prostheses include screws that have a first end threaded for engagement into the bone and a second end configured for holding or supporting the prosthetic component. The configuration of each end has taken many different forms. For example, U.S. Pat. No. 4,484,570 shows a screw with a threaded end that is hollow and that has apertures extending radially therethrough. The threads are intended to grippingly engage adjacent bone stock. The apertures in the hollow side wall are intended to permit bone ingrowth. In some instances, the opposed second end of the bone screw also is threaded to enable threaded engagement with another portion of the prosthesis. Such dual threaded bone screws are shown, for example, in U.S. Pat. No. 5,019,079 and U.S. Pat. No. 5,217,462. Other prior art bone screws merely provide an enlarged head that will engage areas of the prosthetic component adjacent the apertures for holding the prosthetic component against the bone.
Some orthopedic prostheses utilize bone cement for affixing the prosthetic component to the bone. The bone cement typically is applied at the interface between the bone and the prior art prosthetic components. The prosthetic component is then urged into place. Excess bone cement will ooze from the interface between the bone and the prosthetic component and can be removed intraoperatively.
At least one prior art prosthetic component uses a combination of bone screws, foil and bone tissue to cap a damaged region of bone. In particular, U.S. Pat. No. 5,196,016 shows bone screws having a first threaded end and an opposed smoothly cylindrical end. The threaded end is urged into bone and the smoothly cylindrical end projects from the bone. A foil is wrapped over the projecting ends of the bone screws and edge regions of the foil are then attached to regions of the bone. Areas between the bone and the foil are filled with bone mass that is intended to grow between the bone and the foil. The bone screws and the foil are then removed to leave a regenerated region of bone conforming to the shape defined by the foil. These bone screws can be removed due to the smoothly cylindrical outer surface of portions of the screws projecting from the bone and surrounded by the bone mass. In particular the bone mass will surround but not engage the smooth portion of the screw.
Patients with strong healthy bones generally do not receive prosthetic components. Conversely, many patients receiving prosthetic components have weak, damaged or diseased bones. Consequently, there often is no secure bone stock for the prosthetic component to bear against. Implantation of the prosthetic component, therefore, requires more than a minor resection of a bone surface to match the shape of the prosthetic component. Bone screws can extend from the prosthetic component to a region of bone that is sufficiently healthy to hold the screw. However, bone screws typically are very narrow and can flex under compressive loads transmitted through the prosthetic component and the screw.
Accordingly, it is an object of the subject invention to provide a prosthetic component that can be securely affixed to bones that may have damaged, weak or diseased regions adjacent to the prosthetic component.
It is another object of the subject invention to provide a prosthetic system that is sufficiently strong in response to both compressive and tensile forces.
It is another object of the subject invention to provide a screw for efficiently securing a prosthetic component to a weak, damaged or diseased region of bone.